Multi-Input Multi-Output (MIMO) systems have been used in recent years to increase bandwidth efficiency of wireless communication systems. In such systems, M transmit chains/antennas are utilized at the transmitting end, whereas N receiver chains/antennas are utilized at the receiving end. All transmitting antennas {1, . . . , M} and all receiving antennas {1, . . . , N} are tuned to the same carrier frequency (i.e., channel). In other words, the transmit frequencies {ftx1, . . . , ftxM} and the receive frequencies {frx1, . . . , frxN} are all set to the same frequency.
It has been established that under ideal conditions, MIMO systems exploit spatial diversity and propagation richness to increase throughput capacity by a factor of min (M,N) compared to a comparable single-input single-output (SISO) system. This is achieved by different methods of MIMO processing such as spatial multiplexing and space-time diversity coding. In MIMO implementations today, all available transmit/receive chains are configured to the same carrier frequency (i.e., the same channel). The literature on design, optimization, analysis, and implementation of MIMO systems is based on the above assumption. That's because they attempt at optimizing the spectrum efficiency over the occupied frequency channel. Additionally, all recent and under-development wireless standards today consider MIMO operations as described above.
MIMO wireless systems use different coding schemes to exploit their multiple transmit/receive antennas to improve performance and throughput. In one usage scenario, space-time diversity coding schemes are used to encode data streams that are transmitted over different antennas. The primary purpose of space-time diversity coding is to provide spatial diversity gain by transmitting copies of the same data stream over different antennas (hence diversity against channel fading on a set of antennas). In a multiplexing usage scenario, multiple antennas are utilized to transmit multiple data streams over the same frequency channel (hence higher throughput). Similarly, coding schemes designed for spatial multiplexing are used to enable and improve the multiplexing usage scenario.
In scenarios where available spectrum is divided into sub-channels (i.e., K channels of {CH_1, CH_2, . . . , CH_K}), a MIMO system is configured such that all transmit/receive antennas are tuned to one channel out of available channels. There are several limitations and shortcomings with the above mode of operation used in today's MIMO systems. One such limitation is that the increase in capacity by factor of min (M,N) is only achieved if the channel response between different pairs of transmit (TX) and receiver (RX) antennas are uncorrelated. In other words, the channel response between ith transmit antenna and jth receive antenna is uncorrelated with respect to other channel responses for i={1, . . . , M} and j={1, . . . , N}. As the correlation between the propagation channels increases, the promised capacity by MIMO processing rapidly decreases.
Uncorrelated propagation channels are typically present in scatterer-rich environment and where there is no strong dominant line-of-sight. For instance, the promised capacity substantially decreases in the following common cases: i) where there is a line-of-sight (LOS) between the transmitter and receiver sides, ii) the transmitter and receiver sides are relatively close together, iii) the reflective environment surrounding the link is not scatterer-rich. The ability to transmit min(M,N) streams of data over the same frequency band relies heavily on independence between the channels experienced by each data stream. A mathematical analysis where the MIMO channel capacity is characterized as a function of correlation between channels is discussed in Emre Telatar, “Capacity of multi-antenna Gaussian channels,” European Transactions on Telecommunications, vol. 10, num. 6, pp. 585-596, 1999 which is incorporated herein by reference.
Another limitation is in multiple-access scenarios (which is the most common scenario in practice), the available sub-channels {CH_1, CH_2, . . . , CH_K} are typically shared by multiple users, where each user (both transmit/receive sides) is assigned one frequency channel (CH_i, i=1, . . . , K) at any given time period. However, depending on the traffic load and the number of active users accessing the channels in a geographic area, multiple channels {CH_i} could become available at different time periods. These multiple available channels are, however, not assigned to a single user to instantly increase its bandwidth and capacity.